This invention relates to a process for mechanically plating metal articles with cobalt-containing coatings and to a plating composition for use in this process.
Mechanical plating is a known process for applying coatings of ductile metal to articles of iron, steel or other metals. Generally, in this process the articles to be mechanically plated are tumbled in a rotating drum in a slurry with a metal powder and an impact media, such as tiny glass beads. During the tumbling operation the impact media actually peen the metal particles onto the articles which are to be plated, so that the ductile metal particles are cold welded to the article. The slurry may also contain various dispersing agents and additives which affect the plating process.
Examples of mechanical plating are disclosed in the following U.S. patents: Songas, U.S. Pat. No. 3,013,892; Portberg et al., U.S. Pat. No. 3,251,711; Clayton, U.S. Pat. No. 3,479,209; Coch, U.S. Pat. No. 4,654,230; Grunwald et al., U.S. Pat. No. 4,800,132; Clayton, U.S. Pat. No. 4,832,985; Clayton, U.S. Pat. No. 4,849,258; Whitmore, U.S. Pat. No. 4,868,066; and Coch et al., U.S. Pat. No. 4,880,132.
Mechanical plating has the advantages that it is less likely to result in hydrogen embrittlement of the plated articles and also that the energy costs involved in carrying out mechanical plating are generally comparatively low. Accordingly, mechanical plating has found increasing use for plating small metal articles such as screws, bolts, nails, nuts, washers, lock-rings, stampings and the like. However, known mechanical plating processes are subject to some serious limitations.
In general, successful results have only been obtained with the most ductile metals such as zinc, tin, copper, cadmium, lead, aluminum, silver or gold. Although attempts have been made to produce coatings containing less-ductile metals, such as nickel or cobalt, the results have usually been less than fully satisfactory.
Mechanically plated coatings such as zinc/cadmium or tin/cadmium provide a bright finish with good corrosion resistance. However, metals such as cadmium and lead have adverse health and environmental effects. Cadmium is a known human carcinogen, and lead is neurotoxin. Both metals persist in the environment. Therefore, it would be very desirable to be able to provide mechanically plated coatings which avoid the use of metals such as cadmium and lead and yet still show good corrosion resistance and an acceptably bright finish.
Accordingly, there has remained a need in the art for improved processes and compositions for mechanically plating metal articles.